This invention relates generally to improvements in methods and apparatus for forming three-dimensional objects from a fluid-like medium and, more particularly, to a new and improved stereolithography system involving the application of enhanced data manipulation and lithographic techniques to production of three-dimensional objects, whereby such objects can be formed more rapidly, reliably, accurately and economically, and with reduced stress and curl.
In recent years, "stereolithography" systems, such as those described in U.S. Pat. No. 4,575,330 entitled "Apparatus For Production Of Three-Dimensional Objects By Stereolithography," and U.S. Pat. No. 4,929,402 entitled "Methods For Production of Three-Dimensional Objects By Stereolithography" which are hereby fully incorporated by reference herein as though set forth in full, have come into use. Basically, as practiced in one embodiment, stereolithography is a method for automatically building complex plastic parts by successively printing cross-sections or layers of photopolymer (such as liquid plastic) on top of each other until all of the thin layers are formed and joined together to form a whole part. With this technology, the parts are literally grown in a vat of photopolymer liquid plastic. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form and for making prototypes.
Photocurable polymers change from liquid to solid in the presence of light and their photospeed with ultraviolet light (UV) is fast enough to make them practical building materials. The material that is not polymerized when a part is made is still usable and remains in the vat as successive parts are made. An ultraviolet (UV) laser generates a small intense spot of UV. This spot is guided across the liquid surface with a galvanometer mirror X-Y scanner. The scanner is driven by computer generated vectors or the like. Precise complex patterns can be rapidly produced with this technique.
The laser scanner, photopolymer vat, elevator along with a controlling computer and a separate or combined slicing computer combine together to form a stereolithography apparatus, referred to as an "SLA." An SLA is programmed to automatically make a plastic part by drawing a cross section at a time, and building the part up layer by layer.
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link between CAD/CAM. However, such systems have encountered difficulties relating to shrinkage, stress, curl and other distortions, as well as resolution, accuracy and difficulties in producing certain object shapes.
Objects made using stereolithography tend to distort when the materials used change density between the untransformed state (e.g. liquid state) and the transformed state (e.g. solid state). This density change causes material shrinkage or expansion generating stress in a part as it is formed such that lower layers or adjacent structures tend to "curl" giving an overall distortion to the part. Materials with less density change exhibit less curl, but many materials that are otherwise useful for stereolithography have high shrinkage.
The term "curl" is used to describe an effect similar to that found when applying coatings to such things as paper. When a sheet of paper is coated with a substance that shrinks, it curls up toward the coating. This is because the coating both shrinks and sticks to the sheet, and exerts a pulling force on the top but not on the bottom of the sheet. A sheet of paper has insufficient restraining force to resist this pulling. The same phenomenon occurs when a photopolymer is cured on top of a thin layer of already cured photopolymer. The "curl" effect limits the accuracy of the object formation by stereolithography.
Material shrinkage is a common problem with polymer materials, and with fabrication methods (such as plastic molding) that use these materials. However, stereolithography is a new technology, and the problems associated with distortion due to shrinkage are only beginning to be addressed. Additional details about stereolithography are available in the following co-pending U.S. patents and U.S. Patent applications, all of which, including appendices, are hereby fully incorporated by reference herein as though set forth in full:
______________________________________ APPLI- CATION SER. NO. FILING DATE INVENTORS STATUS ______________________________________ 07/182,801 April 18, Hull et al. U.S. Pat. No. 1988 4,999,143 07/182,830 April 18, Hull et al. U.S. Pat. No. 1988 5,059,359 07/183,015 April 18, Smalley U.S. Pat. No. 1988 5,015,424 07/183,016 April 18, Modrek U.S. Pat. No. 1988 4,996,010 07/268,429 November 8, Modrek, et U.S. Pat. No. 1988 al. 5,076,974 07/268,816 November 8, Spence U.S. Pat. No. 1988 5,058,988 07/268,837 November 8, Spence, et U.S. Pat. No. 1988 al. 5,123,734 07/268,907 November 8, Spence et U.S. Pat. No. 1988 al. 5,059,021 07/331,644 March 31, Hull et al. Allowed 1989 07/365,444 June 12, Leyden et Allowed 1989 al. 07/339,246 April 7, Hull et al. U.S. Pat. No. 1989 5,104,592 07/428,492 October 27, Vorgitch et Abandoned 1989 al. 07/429,435 October 30, Smalley et al. Allowed 1989 07/429,911 October 27, Spence et Allowed 1989 al. 07/515,479 April 27, Almquist et Allowed 1990 al. ______________________________________
Additional details of stereolithography are also available in three related applications. The disclosures of these three additional applications are hereby fully incorporated by reference herein as though set forth in full.
The first of these is U.S. patent application Ser. No. 07/606,802 entitled "Simultaneous Multiple Layer Curing for Forming Three-Dimensional Objects," filed by Smalley et al. This application describes methods of building high resolution objects from traditionally low-resolution combinations of building materials and synergistic stimulation. These combinations result in a minimum effective cure depth which is typically too deep to form the thin layers required for high resolution objects. The objective of this referenced invention is accomplished by delaying the exposure of a particular area on a cross-section that would negatively impact resolution if immediately cured along with formation of the rest of the cross-section. Resolution may be negatively impacted by the cure depth involved, for example, when material below the cross-section is inadvertently cured upon exposure of the area. Therefore, to preserve resolution exposure of the particular area is delayed until corresponding areas on higher level cross-sections are subsequently exposed. The higher level cross-sections are chosen such that the cure depth is deep enough to cure the desired volumes (areas) without inadvertently curing material on lower cross-sections. The processes of this referenced application are similar to those of the instant invention in their curing of portions of lower layers from higher layers by application of cure depths substantially greater than one layer thickness.
The second of these is U.S. patent application Ser. No. 07/605,979 entitled "Improved Surface Resolution in Three-Dimensional Objects by Inclusion of Thin Fill Layers," filed by Smalley et al. This application describes methods for forming high resolution objects by filling surface discontinuities inherent in three-dimensional objects formed from stereolithography with thin fill layers.
The third of these is U.S. patent application Ser. No. 07/606,191 entitled "Boolean Layer Comparison Slice," filed by Snead et al. This application describes the use of Boolean operations in determining (1) which portions of a layer continue from a previous layer through a present layer and through the next successive layer and (2) which portions of the layers are up-facing or down-facing or both. Therefore, this application describes methods and apparatus for comparing initial data associated with each layer and for comparing such data between layers to form resulting data that will be used in the process of physically reproducing the object. Additionally, this application describes the use of such operations to yield the appropriately sized objects (e.g. undersized or oversized). The methods of multiple layer curl balancing, to be described herein are readily implemented following the teachings of this referenced application. The three preceding applications and all appendices and patents mentioned therein are hereby fully incorporated herein by reference as though set forth in full.
As more fully described in several of the references previously listed or discussed, methods and apparatus have been developed to reduce curl which utilize creative stereolithographic building techniques. These building methods include, but are not limited to three concepts or techniques known as the brick and mortar technique (sometimes called tiling), the multipass technique, and the riveting technique.
The brick and mortar curl reduction technique involves curing a fluid-like material such as a liquid resin, transforming a powdered material by sintering or by using a binder to form successive solid portions or bricks that have breaks therebetween and adhere to a lower previously cured layer of material. Forming the layer as a series of isolated cured regions allows the layer to be cured to isolate stress along the layer. In some embodiments, the breaks between the successive bricks are filled with liquid resin and analogized to mortar. The bricks are cured with greater exposure than the mortar. Since the mortar is cured less than the bricks, less shrinkage occurs along this region. Stress is isolated to a greater or lesser extent in the regions of the individual bricks depending on the exposure given to the mortar regions. Thus, curling is significantly isolated to limited regions along the length of a layer.
Although the Brick Concept results in significant curl reduction, it may also result in the creation of relatively weaker parts and a rough surface finish due to the non-curing or limited curing of the mortar. In some instances, post process filling may also be required to fill holes. In other instances, if the mortar is subsequently exposed in order to improve strength, a substantial amount of the curl may be reintroduced.
The multipass curl reduction technique involves incrementally curing a layer of building material (e.g. liquid resin) to a particular depth through multiple passes of the synergistic stimulation (e.g. a UV laser beam) over the building material. The resin is cured such that it does not adhere to an adjacent already-cured lower layer on the initial pass of the UV laser. Instead, adhesion is achieved at a later pass or over the course of several later passes.
Multipass reduces curl in two ways. First, it cures a layer incrementally and enables the top portions of a layer to cure without transmitting stress (by inducing a torque) to previously cured layers. The layers are cured almost to the point of adhering to each other (being separated by a distance of only a few mils). The layers are then adhered to each other in a subsequent or "adhesion" pass. Since only a small amount of the lower portion of the layer is cured on the final pass, less shrinkage is encountered than when curing on a single pass, and therefore less stress is transmitted to the lower layer.
Second, the multipass technique reduces curl because the adhesion pass cures resin sandwiched between a rigid already-cured lower layer, and a rigid (but generally less rigid) already-cured portion of the upper layer. Thus, the curing of this resin will simultaneously introduce stresses to both the upper and lower cured layers, which will tend to cancel each other out.
Although the multipass technique effectively reduces curl, it still may result in some upward curling. Also, this technique may result in what is known as birdnesting which is a distortion that can occur if there are significant delays between the multiple passes. Birdnesting occurs, for example, when resin cured on a particular pass is allowed to float for a long period of time on the surface of a liquid resin before additional passes adhere this cured resin to the layer below. If the delay is long enough, the cured resin floating on the surface of the resin can migrate out of alignment with the lower layer before it is adhered thereto.
In the riveting technique, a cross-section (layer) or portion thereof is transformed using exposures small enough to prevent a newly formed layer from curing deep enough to adhere to a previously formed layer. Next various locations on the newly formed cross-section that overlap cured or transformed locations of the previously formed cross-section are given sufficient additional exposure to extend the cure depth from the lower surface of the newly formed layer to the previously formed layer to adhere them together. Each location that receives an additional exposure comprises only a small cured area. This technique results in a negligible amount of stress transmittal between the layers, thus resulting in little or no curl. For example, a layer of a rail may be built with two parallel walls which are held together by internal hatching vectors, wherein both the walls and hatch are given small enough exposures to prevent the present layer from adhering to the previous layer. The hatch vectors that are then at least partially reexposed form cure to a depth deep enough to cause the layers to adhere at these points of additional exposure (known as rivets) and thereby bond the layers together.
The riveting method is very successful in reducing curl because the rivets tend to induce shrinkage in limited regions along the layer. The shrinkage encountered is less than usual since most of the material being cured is being cured, for a second time and has already undergone a substantial portion of the shrinking process. However, due to the many spaces or breaks between the rivets and the lower structural strength of the individual layers due to their shallow cure depth, this method results in relatively weak parts and layers. Therefore, there are sometimes a number of structural integrity considerations that must be made when building parts.
Although the three techniques described successfully and effectively reduce stress and curl, it must be recognized that, in general, each given application involves a trade-off between structural strength and curl. That is, the higher the structural strength required for a particular application, the more curling that will occur between layers.
It is further noted that although the above-mentioned descriptions of curl and curl reduction techniques are presented with respect to upward vertical curl they are also applicable to other forms of curl, including downward curl when a part is being built upside down, sideways curl when a part is being built sideways, and various forms of horizontal curl (curl in a plane perpendicular to the building axis when lines of transformed material are formed in contact with each other on a single layer).